Abstract: Understanding hydrodynamic process is key to study and improve the environment problems in reservoirs. Based on high-frequency field monitoring of flow and water level in the Xiangxi Bay (XXB) of the Three Gorges Reservoir(TGR) during release period (May 2019) and filling period (September 2019), the characteristics of flow oscillation in XXB are analyzed. The results are as follows. ① The barotropic flow oscillation with fundamental Helmholtz mode is a universal hydrodynamic process in the TGR. ② This oscillation has a period of nearly 2 h, causes strong periodic flows with an amplitude up to ±0.05—0.1 m/s and periodic water level fluctuation with an amplitude up to 0.2 m at the upstream of Xiangxi Bay. ③ The tributary oscillation is driven by the gravity wave at the mouth of tributary, which is induced by the diurnal discharge operation of TGR. This study further reveals the response mechanism of the hydrodynamics in a tributary bay to the discharge operation in TGR. It also provides a new perspective for the future basic and applied research in the reservoir operation.